Optical probes such as those used for Raman detection are increasingly being employed in on-line process-control applications. In a typical configuration, this requires that at least the sampling optic be immersed directly into a process stream. In many cases, the process stream contains materials which tend to coat the optic, thereby reducing or completely eliminating the ability to collect data.
Many solutions have been tried to compensate for, or to remove, such coatings, including ultrasonic cleaners, spray jets, mechanical “windshield” wipers, and even arrangements which automatically retract, clean and reinsert the probe. Each of these approaches has distinct disadvantages. Ultrasonic cleaners have difficulty maintaining sufficient energy density at the optic, and do not operate well in viscous fluids. Spray jets are affected by process flows, and require large volumes of solvent. Mechanical approaches introduce unwanted sealing requirements into the process, and may pose safety and/or reliability problems.